Myelodysplastic Syndromes (MDS) are clonal hematopoietic stem cell (HSC) disorders in which dysregulated hematopoiesis leads to cytopenias of the peripheral blood and are associated with an increased propensity to develop Acute Myeloid Leukemia (AML). The current model of MDS explains that pre-malignant HSCs acquire a clonal advantage and outcompete healthy HSCs leading to ineffective hematopoiesis. We have established that expression of TNFAIP3 (A20), a dual ubiquitin-editing protein involved in the negative feedback regulation of immune signaling, is downregulated in HSCs isolated from MDS patients. Furthermore, our preliminary data shows that deletion of A20 in mouse HSCs results in impairment of hematopoiesis in vivo followed by a fatal hematologic malignancy. This findings suggest that A20 is essential for HSC function; however, the mechanism by which A20 preserves normal hematopoiesis and prevents MDS remains unknown. The goal of this research project is to understand how aberrant ubiquitin-editing function of A20 contributes to hematopoietic stress and initiation of MDS. We hypothesize that loss of A20-mediated ubiquitin-editing drives MDS and impairs HSC survival by perturbed ubiquitin signaling. To elucidate this proposed mechanism and test our hypothesis, two aims have been developed: Aim 1 will establish the consequences of A20 loss on HSC function and progression to MDS and AML; and, Aim 2 will determine the requirement of either the deubiquitination domain or the E3 ligase domain of A20 on HSC function. Our studies will reveal A20's implication in the initiation and progression of MDS. Understanding the mechanism of A20 loss on MDS progression will enable future studies to identify therapeutic targets for MDS patients.